Rare earth magnet is based on intermetallic compound R2T14B, thereinto, R is rare earth element, T is iron or transition metal element replacing iron or part of iron, B is boron; Rare earth magnet is called the king of the magnet as its excellent magnetic properties, the maximum magnetic energy product (BH)max is ten times higher than that of the ferrite magnet (Ferrite); besides, the maximum operation temperature of the rare earth magnet may reach 200° C., which has an excellent machining property, a hard quality, a stable performance, a high cost performance and a wide applicability.
There are two types of rare earth magnets depending on the manufacturing method: one is sintered magnet and the other one is bonded magnet. The sintered magnet of which has wider applications. In the conventional technique, the process of sintering the rare earth magnet is mainly performed as follows: raw material preparing →melting→casting→hydrogen decrepitation (HD)→jet milling (JM)→compacting under a magnetic field→sintering→heat treatment→magnetic property evaluation→oxygen content evaluation of the sintered magnet→machining→surface treatment and so on.
The development history of the sintered rare earth magnet cannot be overly summarized in a word that it is the developing of improving the content rate of the main phase and reducing the constitute of the rare earth. Recently, to improve (BH)max and coercivity, the integral anti-oxidization technique of the manufacturing method is developing continuously, so the oxygen content of the sintered magnet can be reduced to below 2500 ppm at present; however, if the oxygen content of the sintered magnet is too low, the affects of some unstable factors like micro-constituent fluctuation or infiltration of impurity during the process is amplified, so that it results in over sintering, abnormal grain growth (AGG), low coercivity, low squareness, low heat resistance property and so on.
To improve the coercivity and squareness of the magnet and solve the problem of low heat resistance, it is common to perform grain boundary diffusion with the heavy rare earth elements such as Dy, Tb, Ho and so on to the sintered Nd—Fe—B magnet, the grain boundary diffusion is generally performed after the machining process before the surface treatment process. The grain boundary diffusion method is a method of diffusing Dy, Tb and other heavy rare earth elements in the grain boundary of the sintered magnet, the method comprises the steps in accordance with 1) to 3):
1) coating the rare earth fluoride (DyF3, TbF3), rare earth oxide (Dy2O3, Tb2O3) and other powder on the surface of the sintered magnet, then performing grain boundary diffusion of the elements Dy, Tb to the magnet at a temperature of 700° C.˜900° C.;
2) coating method of rich heavy rare earth alloy powder: coating DyH2 powder, TbH2 powder, (Dy or Tb)—Co—No—Al metallic compound powder, then performing grain boundary diffusion of DY, Tb and other elements to the magnet at a temperature of 700° C.˜900° C.;
3) evaporation method: using high temperature evaporation source to generate Dy, Tb and other heavy rare earth metal vapor, then performing grain boundary diffusion of DY, Tb and other elements to the magnet at a temperature of 700° C.˜900° C.
By the grain boundary diffusion method, the values of Br, (BH)max of the magnet remain unchanged essentially, the value of coercivity is increased to about 7 kOe, and the value of the heat resistance of the magnet is raised about 40° C.
The above mentioned method performs grain boundary diffusion under the temperature condition of 700° C.˜900° C., although the value of coercivity is increased, there are still some problems:
1. the diffusion takes a long time, for example, it may take 48 hours for diffusing the heavy rare earth element to the center of a magnet with a thickness of 10 mm, however, it may not ensure 48 hours of diffusion time in mass production because it has to increase the manufacturing efficiency by shortening the diffusion time; therefore, the heavy rare earth element (Dy, Tb, Ho or other elements) may not be sufficiently diffused to the center of the magnet, and the heat resistance of the magnet may not be sufficiently improved;
2. the magnet may react with the placement and the rule, therefore the surface of the magnet material would be scratched, and the cost of the rule consumption is high;
3. the magnet may have a low oxygen content, consequently the oxidation may not be evenly distributed through the inside and outside of the magnet, the oxidation film may not be evenly distributed, and the magnet may easily deform (bend) after the RH diffusion.